A problem that has plagued manufacturers and users of optical devices for centuries is focusing all of the different wavelengths of visible light onto the same point to produce a sharp image. When this light is not focused to the same point, what is known as chromatic aberration is produced (this is apparent as a rainbow fringe around high contrast borders and objects, or as a blurrier image in BW photography). Modern lens manufacturers have however found ways to produce relatively simple lenses with minimal chromatic aberration, however it can often still be seen towards the edge of the lens/at the border of a photograph.

A quirk of lights differing focal points is very crutially notable in infrared photography. Focus must be shifted from that of visble light when preparing to take an infrared photograph. Older lenses are often equiped with a handy red line or dot marked off to the right of the focus line that shows how far you must shift focus for infrared light to create a sharp image in camera. 

Below is a demonstration of the amount of focus shift that occurs, along with some obligator IR color shifts. The visible light focuses on the Timpani, while infrared light is focused on the marimba in the background.